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Different Effects of Tropical Cyclones Generated in the South China Sea and the Northwest Pacific on the Summer South China Sea Circulation

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Different Effects of Tropical Cyclones Generated in the South China Sea and the Northwest Pacific on the Summer South China Sea Circulation J Oceanogr (2011) 67:347–355 DOI 10.1007/s10872-011-0044-1 ORIGINAL ARTICLE Different effects of tropical cyclones generated in the South China Sea and the northwest Pacific on the summer South China Sea circulation Zheng Ling • Guihua Wang • Chunzai Wang • Zhi-Song Fan Received: 2 October 2010 / Revised: 17 May 2011 / Accepted: 24 May 2011 / Published online: 17 June 2011 Ó The Oceanographic Society of Japan and Springer 2011 Abstract Tropical cyclones (TCs) that affect the South southwesterly and northeasterly winds prevail in the SCS in China Sea (SCS) can be generated in either the SCS or the summer and winter, respectively. The southwesterly wind northwestern Pacific (NWP). Using satellite measurements, usually occurs around mid-May in the southern and central the Sverdrup theory and a 1.5-layer nonlinear reduced SCS, then expands to the entire SCS in June, and matures gravity model, the present paper investigates the effects of from July to August. The strong northeasterly winter SCS and NWP TCs on the summer SCS upper layer ocean monsoon first appears over its northern part in September, circulation. Both SCS and NWP TCs enhance the summer reaching its central part in October and covering the entire mean circulation pattern of the cyclonic gyre in the SCS in November, and then gradually diminishing in April. northern SCS and the anti-cyclonic gyre in the southern The SCS is also an ocean basin that is often affected by SCS. However, the effect of SCS TCs is much larger than tropical cyclones (TCs) generated either over the north- that of NWP TCs, although the number of SCS TCs is western Pacific (NWP) or the SCS (Wang et al. 2007). As smaller than NWP TCs. This is because the SCS TCs- in many studies, we define TC as a tropical mesoscale induced wind stress curl pattern is favorable for enhancing cyclonic weather system with its strength of a tropical the summer SCS mean circulation. storm or stronger (i.e., typhoon). Our TC dataset shows that, among the summer (winter) average of 5.3 (3) TCs Keywords Upper layer circulation Á Tropical cyclones Á passing through the SCS, about 1.5 (0.25) TCs are origi- South China Sea Á The northwestern Pacific nated or generated within the SCS and the remaining TCs are originated from the NWP. Since large-scale upper layer ocean circulation in the 1 Introduction SCS is mainly driven by the surface wind, both the mon- soon-related and TC-induced winds should play a role in The South China Sea (SCS) is a semi-enclosed ocean basin SCS circulation. How the SCS upper layer responds to the temporally forced by the pronounced monsoon wind. The monsoon has been well studied (Liu et al. 2001; Wang et al. 2006). It has been shown that a winter basin-wide cyclonic gyre with two cyclonic cells is forced by the Z. Ling Á Z.-S. Fan College of Physical and Environmental Oceanography, winter monsoon, while a weakened cyclonic gyre north of Ocean University of China, Qingdao, China about 12°N and a strong anti-cyclonic gyre south of about 12°N are produced by the summer monsoon. Associated & Z. Ling Á G. Wang ( ) with these gyres are strong western boundary currents. In State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, SOA, Hangzhou, China winter, a southward jet flows along the entire western e-mail: [email protected] boundary (Wrytki 1961; Liu et al. 2001; Wang et al. 2008). In summer, there is a northward jet flowing along the C. Wang western boundary in the southern SCS (Xu et al. 1982). Physical Oceanography Division, NOAA/Atlantic Oceanographic and Meteorological Laboratory, The jet apparently veers eastward off central Vietnam near Miami, FL, USA 12°N (Xu et al. 1982; Xie et al. 2003; Wang et al. 2003, 123 348 Z. Ling et al. 2006). To the north, there is a narrow southwestern speed. We choose all TCs that could have an impact on the jet along the continental slope in the northwest SCS (Qu SCS, defined for this purpose as the area of 0–22.5°N, 2000; Metzger and Hurlburt 2001;Su2004). 98.5–120.5°E. However, there are few studies that have examined the The QuikSCAT wind dataset we used is from the Jet SCS response to TC-induced wind. Chu et al. (1999) used Propulsion Laboratory. The spatial resolution of Quik- the Princeton Ocean Model to investigate the SCS response SCAT wind dataset is 0.25° latitude by 0.25° longitude and to TC ‘Ernie’ and studied the sea surface temperature the time resolution is 1 day. To evaluate the QuikSCAT cooling to the right of TC track and a very strong divergent wind data quality, we compared the QuikSCAT wind in the upper layer current. Lin et al. (2003) combined several SCS with the observed winds from two islands during May datasets to show the effects of TCs on chlorophyll and 2005–December 2006. The root mean square errors marine life. Recently, Wang et al. (2009) investigated the between the QuikSCAT and island observations for wind seasonal variability of SCS ocean circulation induced by speed and wind direction are 1.65 m/s and 27.8°, respec- TCs using satellite QuikSCAT wind field data and a tively. These values are consistent with the results of Liu reduced gravity model. They found that TCs can affect and He (2003) who compared the QuikSCAT wind in the both large-scale and mesoscale SCS ocean circulation. As SCS with the observed wind on seven islands. As dem- stated earlier, TCs that affect the SCS can be formed in onstrated in previous studies, the QuikSCAT wind over the either the SCS or the NWP. However, whether TCs gen- SCS is useful to simulate SCS circulation (Xie et al. 2003; erated in the SCS and the NWP have different effects on Wang et al. 2006, 2008), although the QuikSCAT wind SCS circulation is unknown. The purpose of the present shows some differences from island observed wind. paper is to answer and address this question. Because there To examine the effect of TCs on SCS ocean circulation, are few SCS TCs during the northeasterly monsoon period, we reconstructed four sets of wind products from the as shown in Table 1, NWP TCs dominate the influence of QuikSCAT wind dataset as follows: TCs on the mean SCS circulation with an enhancement of A. The original QuikSCAT wind data. the basin-wide cyclonic flow in this period. Thus, this study B. The wind data excluding all TCs. will focus on different effects of TCs generated in the SCS C. The wind data excluding TCs generated over the and the NWP on the summer SCS circulation during the NWP. southwesterly monsoon period. D. The wind data excluding TCs generated over the SCS. We will first use the QuikSCAT wind, one of the best wind products available, to estimate the wind stress curls Hereinafter, we will use A, B, C and D to denote the induced by SCS and NWP generated TCs. A simple model four wind forcings, respectively. Note that a simple linear of the Sverdrup theory and a 1.5-layer reduced gravity interpolation in time is used to reconstruct the wind forcing model are then used to investigate the effects of TCs for those periods during which we exclude TCs. For generated in the SCS and NWP on SCS ocean circulation. example, if a TC occurs from 6 to 8 June, we remove the 3 days’ original wind fields first, and then use the wind field on 5 and 9 June to obtain the wind field for the 2 Data removed 3 days with the linear interpolation method. The simple method is very effective in excluding TCs, but The TC dataset is from the Japanese Meteorological leaving the background wind field (the monsoon wind). Agency. The cover period of the dataset is from January A multiple altimeter dataset from the US Naval 2000 to December 2007. The starting year of 2000 is Research Laboratory, derived from TOPEX/POSEIDON, chosen because it is the beginning of a complete year for Jason, ERS and Geosat Follow-On (GFO), is used to val- the QuikSCAT wind data. The TC dataset includes idate our model results. The spatial resolution of the dataset 6-hourly positions of each TC, its impact radius, and its is 0.25° 9 0.25° and the time resolution is daily within the associated minimum center pressure and maximum wind period from year 2000 to 2007. Table 1 The number of tropical cyclones which affect the South China Sea from 2000 to 2007 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SCS tropical cyclones 0 0 0 0 4 3 4 6 5 1 0 1 NWP tropical cyclones 0 0 1 1 2 5 11 13 9 7 12 3 Total 0 0 1 1 6 8 15 19 14 8 12 4 123 Different effects of tropical cyclones generated in the South China Sea 349 3 Methods 3.1 Sverdrup theory The Sverdrup theory is a primary balance for the large- scale wind-driven ocean circulation (Sverdrup 1947). Many studies have applied it to calculate the large-scale circulations in the Pacific, Atlantic and Indian Oceans. This simple model has also been successfully applied to study SCS circulation (Metzger and Hurlburt 1996; Shaw et al. 1999; Liu et al. 2001). The wind-driven transport stream- function (w) for flow integrated in the entire water column can be estimated by the Sverdrup theory as: ZxE 1 wðxÞ¼ curlðsÞdx; qb x where q is the density of the water, b is the northward Fig.
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